1. Field of the invention
The present invention relates to a magnetic recording medium in the form of disk or continuous tape which comprises a nonmagnetic support and a magnetic recording layer.
2. Description of prior arts
A magnetic recording disk has been recently employed as a recording medium for a computer or a word processer. With the practical use of an electronic camera in recent years, the magnetic recording disk (i.e., magnetic sheet disk) of small size is standardized and has been employed as a recording medium for the electronic camera.
The magnetic recording disk basically comprises a nonmagnetic support and a magnetic recording layer provided on a surface of the support, and has a similar construction of layers to that of a magnetic recording tape such as an audio-tape and a video-tape. However, the magnetic recording disk generally has magnetic recording layers on both surfaces of the support, and in this point, the magnetic recording disk is different from the magnetic recording tape. In addition to the difference of shape and construction of layers, there is a large difference in the manner of usage between the magnetic recording disk and the magnetic recording medium in the form of tape (i.e., a magnetic recording tape). Accordingly, the function required for the magnetic recording disk is largely different from that required for the magnetic recording tape.
In more detail, as for the magnetic recording disk, a magnetic head runs repeatedly on the same surface of the disk in contact with the surface of the disk to record and regenerate information, and any information which has been recorded in the disk is not allowed to drop off the disk under such severe conditions. From these viewpoints, it is necessary that the magnetic recording disk has much higher physical resistnace in the running (that is, rotational or revolutional) operation than that of the magnetic recording tape.
Further, as the recording density required for the recording media tends to be prominently higher in recent days, the magnetic recording disk is likewise required to have higher recording density. That is, the magnetic recording disk needs to be further improved in the electromagnetic conversion characteristics thereof.
Since the magnetic sheet disk for the electronic camera is required to show high resolution almost the same as that of the conventional photography using silver salt, the electromagnetic conversion characteristics thereof are particularly required to be highly enhanced, and the allowance for dropping the recorded and regenerated image off the disk (i.e., drop-out of image) is under severe restriction. For instance, the magnetic sheet disk having a disk diameter of approx. 5 cm is required to record an amount of image information almost corresponding to 50 photographs, and hence the magnetic sheet disk for the electronic camera needs to have a recording capacity of much higher density as compared with the conventional magnetic recording disk.
As a measure for improving the electromagnetic characteristics of the magnetic recording disk, Japanese Patent Provisional Publication No. 58(1983)-122623 proposes that a ferromagnetic powder having a coercive force of not less than approx. 1000 Oe and a specific surface area of approx. 25.gtoreq.70 m.sup.2 /g (that is, ferromagnetic metal powder) be employed in a magnetic recording medium.
While a ferromagnetic alloy powder is preferably employed as the ferromagnetic powder for the magnetic recording medium because of its excellent electromagnetic conversion characteristics, there is a drawback that the ferromagnetic alloy powder is low in hardness and easily abraded in the course of running in contact with a magnetic head.
The magnetic sheet disk for the electronic camera is generally employed under such severe conditions that the same surface of the disk is kept in contact with a magnetic head under revolution (or rotation) at a rate of approx. 3,600 rpm. Under such severe conditions, the disk is required to show physical resistance in the rotational operation enough to endure rotations of at least 15 million times. In addition, almost no drop-out of recorded image information is allowed even under such severe conditions. Such high electromagnetic conversion characteristics have been hardly required for the conventional magnetic recording media.